The manufacturing process of a semiconductor device with a submicroscopic pattern, such as an LSI, VLSI, or the like, employs a reduction projection exposure apparatus, which reduces and projects a pattern formed on a mask and transfers it onto a substrate coated with a photosensitive agent. Along with an increase in the integration degree of semiconductor devices, finer patterns have been demanded. Concurrently, with development of resist processes, measures have been taken against exposure apparatuses for miniaturizing patterns.
To improve the resolution of an exposure apparatus, a method of shortening the exposure wavelength or a method of increasing the numerical aperture (NA) of the projection optical system is generally employed.
As for the exposure wavelength, a KrF excimer laser with an oscillation wavelength of 365-nm i-line to recently around 248 nm, and an ArF excimer laser with an oscillation wavelength around 193 nm have been developed. A fluorine (F2) excimer laser with an oscillation wavelength around 157 nm is also under development.
As another technique for increasing the resolution, a projection exposure method using immersion is receiving attention. Conventionally, the space between the final surface of a projection optical system and a substrate (e.g., a wafer) to be exposed is filled with a gas. Immersion performs projection exposure by filling this space with a liquid. For example, assume that pure water (whose refractive index is 1.33 ) is to be provided to the space between a projection optical system and a wafer, and the maximum incident angle of light beams which form an image on the wafer in immersion is equal to that in a conventional method. In this case, the resolution in immersion becomes 1.33 times higher than that in the conventional method, even when a light source having the same wavelength is used in each method. This is equivalent to an increase in NA of the projection optical system in the conventional method by a factor of 1.33 . Immersion makes it possible to obtain a resolution whose NA is 1 or more, which cannot be attained by the conventional method.
To fill the space between the final surface of a projection optical system and a wafer surface, mainly, two types of methods have been proposed.
One of them is a method of placing the final surface of the projection optical system and the entire wafer in a liquid tank. Japanese Patent Laid-Open No. 6-124873 discloses an exposure apparatus using this method.
The other is a method of supplying a liquid only to the space between the projection optical system and the wafer surface, i.e., a local fill method. Publication WO99/49504 discloses an exposure apparatus using this method.
In the method disclosed in Japanese Patent Laid-Open No. 6-124873 , a liquid may splash about when a wafer moves at high velocity, and equipment is required to recover such splashes. Also, micro-bubbles caused by the wavy liquid surface my adversely affect the imaging performance. In addition, this method may increase the complexity and size of the apparatus.
In the method disclosed in WO99/49504 , assume that the gap between a wafer and a projection optical system is small. In this case, even when a nozzle is directed toward the gap, and a liquid is supplied to the gap, the liquid discharged from the nozzle does not flow into the gap, and a gas remains in the gap. For this reason, satisfactory immersion cannot be performed. A liquid having failed to flow into the gap collides with the perimeter of a projection lens and escapes externally. Equipment for recovering the liquid needs to be provided around the perimeter, and the size of the exposure apparatus increases. Even if a liquid can be supplied into the small gap, since the flow resistance inside the gas is larger than that outside the gap, the flow velocity of the liquid discharged from the nozzle is much higher than that in the gap. For this reason, the flow velocity changes excessively at the tip of the nozzle or at a portion where the liquid collides with the perimeter of the projection lens, the flow is greatly disturbed, and air bubbles may be generated. These air bubbles may enter the gap between the projection lens and the wafer, may prevent transmission of light, and may adversely affect the imaging performance of the exposure apparatus.
In the method disclosed in WO99/49504, a liquid supplied onto the wafer needs to be recovered at least for every wafer replacement, and the productivity of the apparatus must be sacrificed to recover the liquid. Recovery of a liquid on the wafer means recovering a liquid below the projection lens. For this reason, a part of the lower surface of the projection lens can get wet at every wafer replacement, another part can be coated with a thin liquid film, and still another part can directly be exposed to the outer air. The environment surrounding the projection lens and wafer contains impurities in larger amounts in comparison with the supplied liquid, and a liquid staying on the lower surface of the projection lens may absorb an impurity contained in the outer air. The liquid staying on the lower surface of the projection lens evaporates to the outer air, and the impurity originally contained in the liquid or an impurity absorbed from the outer air condenses in the liquid. As a result, an impurity may be attached to the surface of the projection lens to cause clouds or the impurity may remain as a residue after the evaporation/drying of the liquid on the surface of the projection lens to cause clouds.